Clean up infected computer from viruses [duplicate]

once viruses infect a system the only real solution is a clean format

This. Once your system is infected, you cannot trust any program it's running not to be interfered with by the virus - including all antivirus software. Theoretically, you could boot an antivirus system from CD, but even then, you can't be sure that the virus hasn't hidden a copy of itself deep in some executable from where it can reinfect the system.

Lately I've heard of viruses that burn themselves in the BIOS, so a clean format might not always work ... how common is this technique? Should I burn a fresh BIOS as well?

Pointless, since you'd be doing so while a hypothetical BIOS-resident virus is running. The only way to be certain would be to remove the BIOS flash chip and rewrite it using dedicated flashing hardware.

But I think BIOS-resident viruses have so far occurred only as proof-of-concept implementations and not been spotted "in the wild". It sounds nasty but is not actually very attractive to virus writers, since it would have to deal with (at least) dozens of different motherboard families, each with its own proprietary BIOS flashing protocol.

To avoid reinstalling, simply pull your infected hard disk and disinfect it from another (known clean) PC, attaching it either with a USB/IDE/SATA converter or directly on the IDE or SATA as a second drive.

This absolutely prevents the virus from defending itself, since it isn't running anything on the new host computer. Be sure to avoid auto-run if using a USB adapter.

I've had complete success with this method with several computers. As for the BIOS virus, I'll believe it when I see it.

I agree with Michael's answer -- clean install is the only way that really makes sense.

On a side note, don't install more than one antivirus. They have to dig deep into the operating system and they usually don't play very well with each other.